This invention relates to coating transfer surfaces on an apparatus which transfers metered amounts of liquid to a substrate, for example to coat or print a workpiece.
Liquid coatings are applied to printed sheets (or webs) for many reasons including: surface protection, scuff resistance, gloss, and faster drying to prevent ink offsetting and smudging. Faster drying is particularly important to improving productivity while printing and coating in-line. Coatings also reduce use of spray powders, which may improve handling of slower drying workpieces, but can be detrimental to the cosmetic appearance, look, and feel of the printed/coated product.
Matte, semi-gloss, medium and high gloss (water base) coatings and the increasingly popular UV (ultra violet) and EB (electron beam) coatings with high-gloss capabilities are commonly used today. Many variables affect gloss, including the coating's thickness, smoothness (lay when drying is initiated), physical properties, and application techniques, as well as the surface upon which coating is applied. All other things being equal, thicker coatings generally provide higher gloss, improved scuff resistance or increased barrier properties.
Another applications for coating printed workpieces involve retail packages known as blister pacs, in which the product is encased in a clear plastic bubble that is adhered to a printed paperboard. Water based adhesive is applied to the paperboard "off-line", in an operation that is performed after, and separate from, printing. A bond or seal is established between the plastic bubble perimeter and a corresponding coated margin on the paperboard by applying pressure and heat to borders of the plastic and to the paperboard. One hundred percent "fiber-tare" (seal) is generally desirable and often times a requirement. A fiber-tare value can be determined in a destructive test, by separating the plastic bubble from the coated board. One hundred percent fiber-tare (a complete seal at the plastic/board interface) is said to be achieved when dried blister coating, ink, and paper coating, if any, are attached to the plastic, leaving only board fibers on the surface of the board in a continuous line extending completely around the periphery of the bubble. It is generally accepted that 100% fiber-tare requires dry coat weights of at least 0.76 to 0.8 lb/1,000 sq.ft., the latter dry coat weight being the equivalent of a wet coat weight of 21/4 lb./1,000 sq.ft., when solids are 35%. In order to apply 21/4 lb./1,000 sq.ft. of wet coating to the blanket, the coating surface and adjacent coater roll(s) often must carry a coat weight in excess of the weight-to-surface area ratio of the workpiece being coated. This relatively heavy layer of wet liquid coating presents certain operational problems (particularly for in-line or other high-speed operations), including drying and slinging (described below).
One effort to apply the blister coating in-line with a printing process involves extending the delivery of the sheet-fed press to provide more drying time. Extending the press delivery does not ameliorate slinging, however, which can occur when a heavy blister coating is transferred to a conventional smooth offset blanket on the blanket cylinder of a sheet-fed press and from there to the board substrate. The smooth blanket is capable of carrying only so much liquid before coating accumulates at the "in" running nip between the coater applicator roll and the blanket and/or at the interface of the surface of the blanket and the sheet. Excess coating then rolls back to, and into, the blanket cylinder gap, where it accumulates and then slings off the blanket cylinder or sheet. Slinging, of course, is a serious problem, and, to avoid slinging, the coat weight applied in-line must be reduced to a level below that which is required to effect complete blister sealing.
U.S. Pat. No. 4,685,414 issued to Mark DiRico entitled "Coating Printed Sheets" discloses metering coating to a blanket by using a textured roll, for example, an engraved anilox roll, with a hard rigid surface of microscopic, uniform, coating-carrying cells.
U.S. Pat. No. 4,779,557 ("Frazzitta") discloses a three-roll coater for coating the blanket of a blanket cylinder on the last unit in an offset sheet-fed printing press. The applicator roller surface and blanket surface of blanket cylinder move in opposite directions and are set in light ("kiss") contact or with a small gap. Frazzitta discloses a "possible elastomeric blanket" having a "pattern of surface depressions" for receiving liquid coating for transfer to individual fed sheets at the nip.
For applications in which adjacent surfaces move in the same direction, heavy coatings are often applied off-line, where the heavy coat weights can be applied and dried at relatively low production speeds through long tunnel dryers. Typically, the equipment required is very expensive and off-line operations are time consuming, so total production cost is increased.